MacLeanDixie HFSBuilding Solid FoundationsFo

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MacLeanDixie HFSBuilding Solid Foundations

Before You Begin

For the preliminary design of a helical ti eback wall, the following informati on should be collected:

• The loading conditi ons, including:

o Boring Logs

o Soil pressure load

o Hydrostati c pressure load if adequate drainage is not provided

o Surcharge loads from buildings, roads, parking lots, heavy equipment, etc. present at the top of the wall

• The design plan for the wall

o Reinforced Cast-in-place Concrete?

o Soldier Pile with lagging?

o Shotcrete?

o Sheet pile?

• Has a global stability analysis been performed?

MacLeanDixie HFSBuilding Solid Foundations

Preliminary Load Calculati ons

1. What type of soil is present? (clay or sand)

2. What is the Standard Penetrati on Test N-value (bpf) of the soil?

3. What is the angle of internal fricti on (φ) for the soil?

If line (A) was clay, enter 0, otherwise Φ = 27 +0.31 x Line2.

4. What is the cohesion (psf) of the soil? If Line 1 is sand, enter 0,

otherwise c = (Line2)/8

5. What is the bearing capacity factor (unitless) of the soil?

If Line 1 is clay, enter 0, otherwise consult the table to the right:

6. What is the unit weight (pcf) of the soil?

7. What is the coeffi cient of acti ve earth pressure (Ka) for the soil?

8. What is the height (feet) of the proposed wall?

9. What is the maximum water level (feet) behind the wall (if no drainage is provided)?

10. Calculate the wall load (psf) due to soil pressure above the water table:

11. Calculate the wall load (psf) due to soil pressure below the water table:

12. Calculate the wall load (psf) due to hydrostati c pressure:

13. Is there an additi onal wall load (psf) due to surcharge pressures?

14. What is the anchor spacing (feet between horizontally adjacent anchor shaft s)?

15. Calculate the required ulti mate load (pounds) on the anchor:

φ Nq

28 10.68501

28.5 11.3158

29 11.98862

29.5 100

30 13.47353

30.5 14.29293

31 15.16906

31.5 16.10645

32 17.11006

32.5 18.18529

33 19.33806

33.5 20.57486

34 21.9028

34.5 23.32966

35 24.86401

35.5 26.51527

36 28.29382

36.5 30.21106

37 32.27962

37.5 34.51343

38 36.92789

Ka = tan2 (45 – )Line 32

(Line8–Line9)2

2X Line6 X Line7=

(Line9)2 x(Line6–62.4)x(Line7)

2

(Line9)2 x 62.4

2

+ (Line6 X Line7 X Line9 X (Line8–Line9)) =

(Line10 X Line11 X Line12 X Line13) X Line14 X 2(safety factor) =

=

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

Anchor Selecti on

Based on the required ultimate load (Line 15) select an anchor shaft from the chart below based on the ultimate ten-sion capacity column:

Then select a helix confi gurati on to support the required load in the available soils. The selecti on of a proper helix confi gurati on for a given loading and soil profi le conditi on is an iterati ve process. The 8”-10”-12” confi gurati on is a good place to start. Then, a larger or smaller confi gurati on can be att empted if either adequate capacity cannot achieved within a reasonable depth or if adequate capacity could be achieved with a smaller helix confi gurati on.

Make sure that soil boring informati on uti lized is taken from soil borings that are within the anti cipated Tieback Ter-minati on Area. Do not uti lize soil boring informati on that is taken from an area that is within the acti ve soil pressure area.

Description DesignationTorsionalCapacity(ft-lbs)

Max DesignCompression

(lbs)

UltimateCompression

(lbs)Max Design

Tension (lbs)Ultimate

Tension (lbs)Kt

1.50" RCS

1.50" RCS (High strength)

1.75" RCS

2.00" RCS

2.875" O.D. Pipe (0.203" wall)

2.875" O.D. Pipe (0.276" wall)

3.500" O.D. Pipe (0.216" wall)

3.500" O.D. Pipe (0.300" wall)

4.500" O.D. Pipe (0.237" wall)

4.500" O.D. Pipe (0.337" wall)

8.625" O.D. Pipe (0.1875" wall)

D6

D7

D10

D15

P28

P28H

P35

P35H

P45

P45H

P8

10

10

10

10

8

8

7

7

6

6

5

5,500

7,000

10,000

15,000

7,500

9,000

11,400

15,000

20,000

26,000

44,500

30,000

35,000

50,000

75,000

40,000

50,000

50,000

70,000

70,000

100,000

120,000

27,500

35,000

50,000

75,000

30,000

36,000

40,000

52,500

60,000

78,000

100,000

60,000

70,000

100,000

150,000

80,000

100,000

100,000

140,000

140,000

200,000

240,000

55,000

35,000

100,000

150,000

60,000

72,000

80,000

105,000

120,000

156,000

200,000Description Configuration

Length In.Configuration

Length Ft.

ApproximateBearing

Plate AreaFt.^2

8” - 10”

10” - 12”

12” - 14”

14” - 14”

8” - 10” - 12”

10” - 12” - 14”

12” - 14” - 14”

24

30

36

42

54

66

78

2.0

2.5

3.0

3.5

4.5

5.5

6.5

0.751

1.184

1.704

1.986

1.462

2.177

2.697

16. Enter the approximate bearing plate area (ft.2):

17. If the soil type (Line 1) is clay, you can compute the geotechnical capacity at this point, otherwise skip to Line 18: Line16 X Line4 X 9 =

18. Enter the installation angle (degrees from horizontal). Depending upon the wall and slope geometry any value from 10 to 30 degrees can be used. We recommend starting at 20 degrees.

19. What is the distance (feet) between the bottom of the wall and the anchor shaft?

20. Determine the required depth (feet) to achieve the required ultimate capacity:

21. Determine the required anchor length (feet) to achieve the required depth:

22. What is the horizontal distance (feet) between the connection point and the failure plane?

Check against the Minimum Embedment DepthDetermine the minimum embedment depth by, first: Finding the distance between the tieback to wall connection point and the failure surface:

23. What is the angle (degrees) of the failure plane?

24. What is the vertical distance (feet) between the connection point and the failure plane?

Line 19 –[Line19 X tan(Line22)/(tan(Line22) + tan(Line 18)]25. What is the straight line distance (feet) between the connection point and the failure plane?

√ (Line23)2 + (Line24)2

26. Then, enter the length (feet) of the selected helix configuration:

27. Then determine the required length (feet) of embedment beyond the failure plane due to helix size:

Enter the largest helix diameter (inches):

28. 4 X Line2 7/12 =

29. Finally, enter any additional length required to reach competent soil:

30. The minimum required length of the anchor is: Line26 + Line28 + Line29 =

31. Finally, choose the greater of the minimum required anchor length (Line 30) and the length required for geotechnical capacity (Line21):

Line15

Line16 X Line5

Line20–(Line8 –Line9)

sin(Line18)

Line19/ (tan(Line22) + tan(Line18))

–62.4 X (Line8–Line9)

Line3

2θ =45 +

Line6–62.4

Disclaimer: These calculations address only the required embedment depth and the required pull-out capacity of the anchor and should be used for preliminary or feasibility analyses only. The internal wall stresses, the global sta-bility of the wall/soil system, the local stresses at the anchor connection points, among other considerations, MUST be evaluated by a licensed structural engineer.

(16)

(17)

(18)

(19)

(20)

(21)

(22)

(23)

(24)

(25)

(26)

(27)

(28)

(29)

Pile Spacing Design Possibiliti es

Tieback Spacing is determined by two factors:

1. The load requirements of the project

2. The designed load capacity of the ti eback. Helical ti eback anchors can only be spaced as close as 3 ti mes the diameter of the largest helix uti lized (measured from edge-of-helix to edge-of-helix).

In the event that the design load requirement exceeds the designed load of the helical ti eback the designer has two opti ons:

1. Reduce the spacing of the helical ti eback anchor within the spacing helical pile spacing limitati ons.

2. Select a ti eback anchor that meets the load capacity requirements.

In the event that the load requirements exceed the spacing capacity of the helical ti eback, a second row of helical ti e-backs must be added to accommodate the load requirement of the wall and the spacing limitati ons of the helical ti e-backs.

LOAD BEARING SOIL

UNSUITABLE SOILUNSUITABLE SOILUNSUITABLE SOIL

ANGLE OF ACTIVE SOIL PRESSURE ( )

Helical Tieback Connecti ons can be developed in the following ways:

1. Tie the helical adapter and plate within the concrete reinforcement of the proposed poured wall.

2. Att ach helical ti eback adapter to the fi nished wall with a plate, washer and nut.

3. Att ach the helical ti eback adapter to a soldier pile systems with a whaler, washer and nut.

4. Att ach the helical ti eback adapter to a fi nished sheet pile wall with either method 2 or 3 listed above.

Corrosion Considerati ons

Please refer to Secti on 11 of the most recent MacLean Dixie Engineering Reference Manual regarding corrosion consider-ati ons of the project.

Repair Application

In addition to use for the construction of new retaining walls, helical tieback anchors can be used to repair existing foundation walls that have been damaged by expansive soils. The picture below illustrates a foundation surrounded by expansive clay soils. In periods of unusually wet weather, the soils can become saturated and expand against the foun-dation walls causing them to bow inwards.

The repair of foundation walls using helical tieback anchors consists of the following steps:

1. A small amount of soil behind the foundation wall is excavated to provide access to the exterior of the wall.

2. A small hole, just large enough to allow the couplings to pass through, is drilled in the wall.

3. The helical anchor is inserted into the basement from the outside, coupling end first.

4. The helical anchor is advanced into the surrounding soil using light, hand-held equipment from the interior of the foundation.

5. When the helical anchor has been advanced to the required depth, a thread bar adapter and wall plate is installed around the anchor on the interior of the wall.

6. Thread bar is inserted into the adapter and the anchor is tensioned to bring the wall back to plumb and then set in place using nuts.

7. Excess thread bar is cut off, if necessary, and the repair is complete.

MacLeanDixie HFSBuilding Solid Foundations